(1) Field of the Invention
The present invention relates to a method for manufacturing a mold for molding an optical element, and in particular to a method for manufacturing a mold through cutting.
(2) Description of the Related Art
Recently, with the increase in demand for high performance and miniaturization of an optical system, microlens arrays with plural lenses are gathering attention as important optical elements. For example, the microlens arrays are used for autofocus cameras. Among such microlens arrays, diffractive lens arrays have especially gathered more attention. This is because each lens of a diffractive lens array has a diffraction grating which has a bladed form (saw-toothed form), and is thin with high diffraction efficiency compared with a normal spherical or aspherical lens. Therefore, a lens shape of the microlens array that is in demand for increasing focusing accuracy shifts from a normal spherical or aspherical shape to a spherical shape (with diffraction element) or aspherical shape (with diffraction element) which has a saw-toothed surface.
Among the methods for manufacturing microlens arrays, methods based on lithography technology and methods based on machining technique are generally known (see Japanese Laid-Open Patent Application No. 2005-173597 and Japanese Laid-Open Patent Application No. 2000-246614).
According to the lithography technology described in Japanese Laid-Open Patent Application No. 2005-173597, a microlens array mold is fabricated by irradiating ultraviolet rays. In addition, the mold is pressed on to a polymer-coated glass board, and by irradiating ultraviolet rays onto the polymer, a microlens array is manufactured.
With the lithography technology described in the above-mentioned document, while a complex fine shape can be simultaneously formed in plural places, it is difficult to freely form a smoothly-curved surface. Thus, with the method based on the lithography technology, it is difficult to manufacture an optical element which satisfies the level of performance, such as a diffractive lens array that has both a smoothly-curved surface and a saw-toothed form and that requires high shaping accuracy.
According to the method based on the machining technique as described in Japanese Laid-Open Patent Application No. 2000-246614, a metal mold, having concave portions formed by cutting a work piece, is fabricated and a microlens array is manufactured using the metal mold. While a work piece is rotated, a cutting tool is moved so that a concave portion is formed in a position that deviates from the center.
With the method described in the above-mentioned document, a curved surface can be freely formed on a work piece, and thus it is possible to mold a microlens array having a smooth surface.
However, a problem is that even with the method described in Japanese Laid-Open Patent Application No. 2000-246614, a concave portion having a fine saw-toothed surface cannot be formed on a work piece. It is therefore not possible to mold a diffractive lens array of high accuracy.
To be more precise, an orbit along which a cutting tool should be moved is expressed as (X, Y, Z)=(R cos(α−θ)+x−tr·cos β, R sin (α−θ), f(x)−d+tr·sin β−tr) in a function using a coordinate x and θ which denotes a rotation angle of the work piece. Note that the coordinate x indicates a coordinate, at which the cutting tool should be placed on a plane to be cut, of the work piece. The coordinate has the center of a concave portion as an origin. A concave portion is thus formed by cutting the work piece while moving the cutting tool along the orbit.
Thus, with the mold manufacturing method described in Japanese Laid-Open Patent Application No. 2000-246614, while the surface of the concave portion can be made smooth due to the movement of the cutting tool along the orbit, it is not possible to form a fine saw-toothed form. Therefore, it is not possible to mold a diffractive lens array of high accuracy.